In a wireless network, a mobile station (MS) may choose to change its serving cell, i.e., radio coverage area of a base station serving the MS, based upon certain criteria, e.g., signal to noise ratio, carrier to interference ratio, block error rate, or for other reasons. This procedure is termed as “Handover”, “Handoff”, “Cell Change”, or “Cell Reselection” in different technologies. Without loosing generality, a cell change may be carried out through an independent decision of the MS or through a decision made by the wireless network in collaboration with the MS. A “hard cell change” occurs when the MS breaks the connection from the serving cell before it establishes a new connection in the target cell. General Packet Radio Service (GPRS) is an example of a technology that utilizes the hard cell change.
During a hard cell change, the mobile station (MS) loses contact with the serving cell. After some time, it establishes a new connection with a target cell. Depending upon the wireless technology, the overall duration that the MS stays disconnected from the network varies. For example, in GPRS the duration is at least two seconds. Due to this temporary outage at the data link layer, the Transmission Control Protocol (TCP) layer may conclude that the in-flight data is lost. TCP's retransmission timeout (RTO) algorithm is designed to handle both lost and delayed data segments the same way; i.e., it does not have the intelligence to distinguish one from the other. After expiration of TCP's retransmission timeout (RTO), TCP retransmits all in-flight data one segment at a time.
In the particular case where the MS is performing a cell change, these retransmissions are bogus and introduce duplicate copies of in-flight data into the network. These retransmissions are termed “spurious” in that the data-in reality-is not lost; it is just extraordinarily delayed. When the MS establishes connection in the target cell, the data buffered at the cell site during the outage is transmitted over the air, in addition to all the data that the TCP sender retransmitted due to the retransmission timeout. This retransmitted data is expensive for a wireless network in terms of poor utilization of airtime, increased buffer occupancy and reduction in overall system performance.
Thus, what is needed is a method and apparatus for preventing a spurious retransmission after a planned interruption of communications, such as a hard cell change.